Apache CouchDB, commonly referred to as CouchDB, is an open source database that focuses on ease of use and on being "a database that completely embraces the web".[1] It is a NoSQL database that uses JSON to store data, JavaScript as its query language using MapReduce, and HTTP for an API.[1] One of its distinguishing features is multi-master replication. CouchDB was first released in 2005 and later became an Apache project in 2008.

Unlike in a relational database, CouchDB does not store data and relationships in tables. Instead, each database is a collection of independent documents. Each document maintains its own data and self-contained schema. An application may access multiple databases, such as one stored on a user's mobile phone and another on a server. Document metadata contains revision information, making it possible to merge any differences that may have occurred while the databases were disconnected.

CouchDB implements a form of Multi-Version Concurrency Control (MVCC) in order to avoid the need to lock the database file during writes. Conflicts are left to the application to resolve. Resolving a conflict generally involves first merging data into one of the documents, then deleting the stale one.[2]

Other features include document-level ACID semantics with eventual consistency, (incremental) MapReduce, and (incremental) replication. Administration is supported with a built-in web application called Futon.

CouchDB (Couch is an acronym for cluster of unreliable commodity hardware)[3] is a project created in April 2005 by Damien Katz, formerLotus Notes developer at IBM. Damien Katz defined it as a "storage system for a large scale object database". His objectives for the database were to become the database of the Internet and that it would be designed from the ground up to serve web applications. He self-funded the project for almost two years and released it as an open source project under the GNU General Public License.

In February 2008, it became an Apache Incubator project and the license was changed to the Apache License.[4] A few months after, it graduated to a top-level project.[5] This led to the first stable version being released in July 2010.[6]

Since the departure of Damien Katz, the Apache CouchDB project has continued, releasing 1.2 in April 2012 and 1.3 in April 2013. In July 2013, the CouchDB community merged the codebase for BigCouch, Cloudant's clustered version of CouchDB, into the Apache project. The BigCouch clustering framework is prepared to be included in an upcoming release of Apache CouchDB.[8]

CouchDB stores data as "documents", as one or more field/value pairs expressed as JSON. Field values can be simple things like strings, numbers, or dates; but ordered lists and associative arrays can also be used. Every document in a CouchDB database has a unique id and there is no required document schema.

ACID Semantics

CouchDB provides ACID semantics.[9] It does this by implementing a form of Multi-Version Concurrency Control, meaning that CouchDB can handle a high volume of concurrent readers and writers without conflict.

Map/Reduce Views and Indexes

The stored data is structured using views. In CouchDB, each view is constructed by a JavaScript function that acts as the Map half of a map/reduce operation. The function takes a document and transforms it into a single value which it returns. CouchDB can index views and keep those indexes updated as documents are added, removed, or updated.

Distributed Architecture with Replication

CouchDB was designed with bi-direction replication (or synchronization) and off-line operation in mind. That means multiple replicas can have their own copies of the same data, modify it, and then sync those changes at a later time.

REST API

All items have a unique URI that gets exposed via HTTP. REST uses the HTTP methods POST, GET, PUT and DELETE for the four basic CRUD (Create, Read, Update, Delete) operations on all resources.

Eventual Consistency

CouchDB guarantees eventual consistency to be able to provide both availability and partition tolerance.

Built for Offline

CouchDB can replicate to devices (like smartphones) that can go offline and handle data sync for you when the device is back online.

Replication and synchronization capabilities of CouchDB make it ideal for using it in mobile devices, where network connection is not guaranteed but the application must keep on working offline.

CouchDB is well suited for applications with accumulating, occasionally changing data, on which pre-defined queries are to be run and where versioning is important (CRM, CMS systems, by example). Master-master replication is an especially interesting feature, allowing easy multi-site deployments.[11]

CouchDB manages a collection of JSON documents. The documents are organised via views. Views are defined with aggregate functions and filters are computed in parallel, much likeMapReduce.

Views are generally stored in the database and their indexes updated continuously. CouchDB supports a view system using external socket servers and a JSON-based protocol.[17] As a consequence, view servers have been developed in a variety of languages (JavaScript is the default, but there are also PHP, Ruby, Python and Erlang).

Applications interact with CouchDB via HTTP. The following demonstrates a few examples using cURL, a command-line utility. These examples assume that CouchDB is running on localhost(127.0.0.1) on port 5984.

Action

Request

Response

Accessing server information

curl http://127.0.0.1:5984/

{"couchdb":"Welcome","version":"1.1.0"}

Creating a database named wiki

curl -X PUT http://127.0.0.1:5984/wiki

{"ok":true}

Attempting to create a second database named wiki

curl -X PUT http://127.0.0.1:5984/wiki

{"error":"file_exists","reason":"The database could not be created, the file already exists."}

International Components for Unicode (ICU) is an open source project of mature C/C++ and Java libraries for Unicode support, software internationalization and software globalization. ICU is widely portable to many operating systems and environments.